In seismic exploration it is known to generate seismic pulses or waves from at least one seismic source and to measure or record the produced wave-field using a plurality of seismic receivers. In this away, reflections, interactions or the like of the seismic pulses with earth formations may be analyzed. A problem in the analyzing of received seismic pulses is the determination and/or filtering of a signature of the signal source so that the source signal can be removed for the received data leaving only the effects of the reflections, interactions and/or the like of the pulses with the earth formations.
Seismic sources are either of the impulse type generating a sharp and sudden peak of wave energy or, alternatively, of the vibrating type generating a sweeping signal of ideally controlled amplitude and frequency spectrum. Marine seismic sources commonly used are impulsive sources comprising a plurality of so-called “air guns” as source elements arranged in an array to produce a combined seismic source which has more desirable characteristics than the individual source elements of the array. Marine vibratory sources exist but are less frequently used.
In marine seismic exploration, an air gun may be used to generate a high pressure air bubble by the sudden discharge of a quantity of high pressure compressed air into the water surrounding the air gun. According to established theoretical knowledge, the elasticity of the air couples with the inertial mass of the surrounding water to produce an oscillating system as the air expands and contracts in size until its energy is dissipated in the water and the bubble reaches its equilibrium volume. These bubble oscillations generate spherical sound waves, which form the seismic signal. As described below in further detail, marine seismic signals may be synchronized so as to enhance the primary pulse in an acquisition method referred to as peak-tuning, or, if the synchronization is tuned to the first bubble, bubble-tuning. The synchronization may alternatively be tuned to any other part of the composite signature of the source.
It is a feature of an array of marine seismic source elements, although not necessarily desirable, that the sound wave transmitted through the body of water is directional, i.e. the shape or signature of the transmitted wave varies with vertical polar angle, and azimuthal polar angle for a source not designed to be azimuthally symmetric (such special sources being described for example in United Kingdom patent GB 2376528). This is seen as a result of: (i) the array having dimensions which are not negligible compared to the wavelengths of sound in the transmitted wave; and (ii) the effect of the free-surface ghost reflection causing each source element to have an approximately equal and opposite virtual image source element in the free-surface mirror when observed at distances far from the source. In a given direction, the signature of a transmitted wave varies in the so-called “near field” as the distance from the array increases until at a sufficient distance from the array, in the so-called “far field”, the shape of the wave remains substantially constant but the amplitude decreases, generally inversely in proportion to the distance from the array. The “far field” of an array or source generally exists at distances greater than D2/λ where D is the dimension of the array and λ is the wavelength.
In U.S. Pat. No. 4,476,553 and in the European Patent EP 0066423, the entire disclosures of which are incorporated by reference herein, the use of an array of near-field hydrophones or pressure sensors arranged to measure the seismic signals generated by an array of air guns producing seismic signals in a body of water is disclosed. As disclosed, each of the hydrophones is placed in the near-field region (as discussed above) no closer than about 1 meter to an associated air gun to provide that the pressure measured at each of the of near-field hydrophones is a linear superposition of the spherical waves from all the oscillating bubbles and their reflections in the free surface. Using the signals obtained by the near-field hydrophones, a synthetic source signal may be derived. This derived synthetic source signal is referred to as a “notional source” and may be used to provide a way of determining the far-field signature of the array of air guns in all angular directions. As observed by Ziolkowski et al. in: Geophysical Prospecting, 1997, 45, 611-639, and in U.S. Pat. No. 4,476,553 col. 1, 11. 46-51 determination of the notional source may be complicated by sea bottom reflections and, as such, accurate marine seismic measurements using the methods described in the patents are confined to deep water seismography.
A variant of the marine seismic source described above is the TRISOR™ source used by WesternGeco Ltd. In the TRISOR™ source, a TRISOR™ marine source controller enables the air gun elements to be synchronized so as to enhance the primary pulse (peak-tuning), or the first bubble (bubble-tuning) or any other part of the composite air gun signature. TRISOR™ also allows acquisition of data from a hydrophone located near to each air gun element. Although commonly referred to as near-field hydrophones (NFH), the trace from each hydrophone is actually in the far-field of the acoustic pressure radiated from the air gun.
Using the TRISOR™ source, the notional source algorithm—as described in Ziolkowski, A., Parkes, G., Hatton, L. and Haugland, T., The Signature of an Air-Gun Array—Computation from Near-Field Measurements including Interactions, Geophysics 47, 1413-1421 (1982) and in European Patent EP 0066423—may be used to compute near-field and far-field signatures of the array as a whole directly below the marine source array, or for any take-off direction in the 2π steradians centered upon the vertical line below the acoustic centre of the array and characterized by vertical polar and azimuthal polar angles. Far-field in this context means a distance which is large compared to the scale length of the marine source array, typically 10-20 m, or its depth of immersion, typically 5-20 m, so that while the composite signature shape is independent of distance, it may still vary with direction.
Other inventions, such as described in U.S. Pat. No. 5,247,486, describe methods for determining a far-field signature of a plurality of seismic source elements by measuring a near-field signature of each seismic source element and interpolating a relationship between the measured near-field signature and a measured far field signature. As disclosed, an initial near-field signature of each seismic source element and an initial far-field signature of the plurality of N seismic source elements are measured simultaneously and an operator is determined from the measurements to calculate subsequent far-field signatures. Similarly, WO-2004068170-A1 discloses a method and apparatus for directional de-signature of a seismic signal. The method includes forming a plurality of far-field signatures representative of a plurality of seismic signals having a plurality of take-off angles, associating a plurality of traces representative of a plurality of reflections of the seismic signals with the plurality of far-field signatures, and forming a plurality of de-signatured traces from the plurality of traces and the plurality of associated far-field signatures. While such inventions provide methods other than the notional source method of removing the source signal from the received seismic signal, they may not be as robust as the notional source methods and they do not address the need for a notional source method that may be used in shallow waters.